Quantum cascade lasers (QCLs) are made of stages each of which is composed of three regions: an electron injector, an active region and an electron extractor. In conventional QCLs, the quantum wells and barriers in all regions are of the same, fixed alloy composition, respectively. The electrons in the injectors of short wavelength (λ˜4.8 μm) QCLs are found to have a higher temperature than that of the lattice. Consequently, such devices optimized for high continuous-wave (CW) power and emitting in the 4.5-5.0 μm range experience substantial electron leakage from the upper laser state and thus, the electro-optical characteristics of the devices are highly sensitive to temperature above room temperature. In particular, these devices exhibit a rapid decrease in the differential quantum efficiency ηd above 300 K (i.e., the characteristic temperature for ηd, T1, has a low value of ˜140 K) and also a relatively fast increase in the threshold-current density, Jth, above 300 K (i.e., the characteristic temperature for Jth, T0, has low values of ˜140 K). As a result, the maximum CW wallplug efficiency ηwp, max (for light emitted at 300 K from the front facet of devices with high-reflectivity-coated back facets) has typical values of ˜12%, far short of the theoretically predicted upper limit of ˜28% (λ=4.6 μm).
State-of-the-art QCLs have been developed which exhibit lower electron leakage and as a consequence higher T1 and T0 values. See J. C. Shin et al, “Ultra-low temperature sensitive deep-well quantum cascade lasers (λ=4.8 μm) via uptapering conduction band edge of injector regions,” Electronics Letters, Jul. 2, 2009, Vol. 45, No. 14. However, the room temperature Jth values for these QCLs are essentially the same as for conventional QCLs because the laser transition efficiency decreases by about 20% compared to conventional QCLs.